The present invention relates to an apparatus for testing a variable delay element utilized for measuring devices such as a pulse generator and a timing generating unit of an LSI tester, and for a timing adjusting circuit in an LSI, and also to an integrated circuit incorporating a testing function for testing delay elements included therein.
The variable delay element has a function to delay a signal in accordance with a arbitrarily set control signal, and is capable of actualizing delay times of kinds the number of which is obtained by normally raising 2 to a control bit number-th power.
A test for a performance of such a variable delay element with respect to each delay time might generally involve the use of an expensive measuring device and a large amount of time
The followings are descriptions of testing methods that have hitherto been practiced.
A first method involves the use of an LSI tester and a separate measuring device, wherein a measurement reference signal with a repetition of "1" and "0" of a logic level is delayed by a variable delay element to the measured, and a rising or falling edge delayed by the variable delay element is detected by use of a reference signal of the measuring device, thereby obtaining a delay quantity.
This technique entails a high-performance LSI tester or a separate measuring device that are capable of generating a fine delay. Further, it is time-consuming to test sequentially the delay elements one by one, and a running cost for the measuring device is high. Moreover, since a boundary of the edge is sought by searching, and hence a timing of the reference signal for searching is controlled. Therefore, a high-performance control system CPU is needed, and besides the test requires the immense time.
A second method is a technique utilized for evaluating an a.c. characteristic of the LSI, wherein a ring oscillator is constructed including the variable delay element to be measured, and an oscillation frequency is measured by a frequency measuring device. That is to say, the delay quantity can be obtained from a variation in the frequency when changing delay setting of the variable delay element.
According to the second method, the test putting an emphasis on only one of the rise and the fall of the waveform is insufficient in terms of a constraint in the circuit of the ring oscillator, and a duty ratio is also required to be measured. Therefore, this involves the use of special circuit and measuring device. Further, a lot of data must be exchanged between a measuring system and a control system, which might take a huge quantity of time.
A third method is a technique by which a pulse signal corresponding to a phase difference between a signal to be tested and a reference signal is obtained, integrated and converted into a voltage, and a voltage value thereof is subjected to analog to digital conversion, thus obtaining a delay quantity.
According to the third method, an integration of pulses during a fixed period is needed for obtaining the voltage value, and a much measuring time is required. Furthermore, the voltage value obtained does not necessarily fall within a convertible range of an A/D converter at all times, and a timing of the reference signal is required to be adjusted corresponding to the delay quantity. Then, the measurement itself becomes complicated.
As described above, any of the delay element testing methods conventionally used has such a defect that the expensive measuring device is needed, and the measurement is complicated enough to take much measuring time.